The field of the invention is magnetic resonance imaging (MRI),and particularly the measurement of hemodynamic parameters such as cerebral blood flow (CBF) and cerebral blood volume (CBV).
Ischemic stroke is one of the leading causes of death and disability. An important treatment for a stroke patient is the use of so-called xe2x80x9cthrombolyticxe2x80x9d agents, or xe2x80x9cclot bustingxe2x80x9d drugs which have been shown to break tip blood clots that are the source of the stroke. These drugs must be administered within three to six hours of the ischernic event and considerable risk of cerebral hemorrhage is associated with their use.
In order to weigh the risks associated with such treatments, methods have been devised to assess the risk of further brain damage if the treatment is not used. One such method is to produce hemodynamically weighted MR images of the brain which reveal the xe2x80x9cat riskxe2x80x9d brain tissue. If produced promptly, these images can assist the physician and the patient in determining whether aggressive treatment methods are warranted. For example, if the hemodynamically weighted MR image reveals that very little further brain tissue is at risk due to the ischemic stoke, then the use of a thrombolytic agent is probably unwarranted. On the other hand, if such images indicate that critical brain tissue is at risk, then the risk of using the thrombolytic agent is warranted.
Hemodynamically weighted MR perfusion images of cerebral blood flow (CBF) may be acquired and used in combination with diffusion-weighted (DWI) MR images to delineate regions of viable brain parenchyma that are at risk of further infarction. The DWI MR image shows ischernic regions where brain cells have died, and the CBF image shows regions with reduced blood flow which indicates at risk tissue. The size of the xe2x80x9cischemic penumbraxe2x80x9d surrounding ischernic tissues is a critical component in evaluating treatment options.
To be of clinical value, the hemodynamically weighted images must be acquired and produced expeditiously. As indicated above, the risks associated with thrombolytic agents increases with time and the prevailing opinion is that such agents should not be used more than 3 to 6 hours after the ischernic event. Diffusion-weighted MR images can be acquired and reconstructed in a matter of minutes using MRI systems as described, for example, in U.S. Pat. Nos. 5,492,123; 6,078,176; 5,671,741 and 5,488,297. The same cannot be said of current methods for producing CBF, MR images.
As described, for example, by K. A. Kemp, et al. xe2x80x9cQuantification of Regional Cerebral Blood Flow and Volume with Dynamic Susceptibility ContrastEnhanced MR Imagingxe2x80x9d Radiology 1994; 193:637-641, it is possible to assess regional cerebral hemodynamics by analyzing NMR signal intensity changes after the first pass of a paramagnetic contrast medium. While passing through the capillary network, a short bolus of contrast material produces local magnetic field inhomogeneities that lead to a reduction in the transverse magnetization relaxation time T2* of the bulk tissue. This susceptibility effect can be recorded by a series of rapid T2*-weighted gradient-echo images which reveal how the NMR signal changes during the first pass of the contrast agent. The resulting NMR signal intensity versus time curves can be converted into contrast agent concentration-time curves. By using the indicator dilution theory, one can then determine two important hemodynamic parameters from these curves: the cerebral blood flow (CBF), known as tissue perfusion, and the cerebral blood volume (CBV). However, the concentration of contrast agent in the arterial blood poolxe2x80x94the xe2x80x9carterial input functionxe2x80x9d (AIF)xe2x80x94must be known if absolute quantification of the CBV and CBF measurements are to be achieved.
Current methods used to measure the AIF require a step in which the operator manually selects a region of interest (ROI), based on anatomic information which depicts an artery. The concentration-time curve from all voxels included in the ROI are then used to calculate the AIF as described, for example, by L. Ostergaard et al xe2x80x9cHigh Resolution Measurement of Cerebral Blood Flow Using Intravascular Tracer Bolus Passages. Part 1: Mathematical Approach and Statistical Analysisxe2x80x9d, Magnetic Resonance In Medicine, 36:715-725 (1996) and B. R. Rosen et al xe2x80x9cPerfusion Imaging With NMR Contrast Agents,xe2x80x9d Magnetic Resonance In Medicine, 14, 249-265 (1990). This presents two problems. First, such a manual operation requires considerable time to perform and this additional time may be critical. And second, it is very difficult to identify a an ROI in a two-dimensional image which is 100% within an artery. Typically, most of the selected an ROI will lie within an artery, but part of the an ROI will lie in surrounding tissues. Because of this xe2x80x9cpartial volume effectxe2x80x9d, the peak value of its concentration-time curve will be less than a true, 100% arterial voxel, and the width of the peak in the concentration-time curve will be increased. This measurement error results in an error in the AIF, which in turn produces an error in the absolute value of the CBF and the CBV.
The present invention relates to the measurement of hemodynamic parameters such as cerebral blood flow, and more particularly, to a method which enables the rapid and automatic production of MR images that more accurately indicate the value of homodynamic parameters throughout a selected region of interest. The present invention is a method for automatically identifying an arterial voxel in a data set acquired during a dynamic contrast-enhanced MR study and calculating therefrom the arterial input function (AIF). Hemodynamic parameters are calculated using the acquired data set and this calculated AIF.
A general object of the invention is to shorten the time needed to produce hemodynamic parameter images. No manual steps are required after acquisition of the data set during the dynamic study. As a result, images can be produced promptly which enable the physician to assess the risks of clinical actions.
Another object of the invention is to provide a more accurate AIF. This is accomplished by first identifying that portion of the dynamic study data set which indicates first passage of the contrast bolus through the region of interest. Within this limited data set, the signal produced by each voxel in the region of interest during first passage is examined to select the one voxel which best exhibits arterial blood flow characteristics. AIF is calculated using the NMR signal produced by this voxel.
The foregoing and other objects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown by way of illustration a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference is made therefore to the claims herein for interpreting the scope of the invention.